Bioadhesive compositions and biomedical electrodes containing them

ABSTRACT

Bioadhesive Compositions which comprise a hydrophobic polymer wherein the concentration of the polymer at the surface of the adhesive is greater than its concentration in the bulk of the adhesive are described; and biomedical electrodes, fixation products and wound dressings containing them.

[0001] This invention relates to bioadhesive compositions, particularlyelectrically conductive hydrogel compositions having bioadhesiveproperties. The invention further relates to biomedical skin electrodesincorporating such hydrogel bioadhesive compositions that areelectrically conductive.

BACKGROUND

[0002] Biomedical skin electrodes are widely used in a variety ofsituations, whenever for example it is required to establish anelectrical connection between the surface of the body of the patient andexternal medical equipment for transmission of electrical signals.

[0003] Modem medicine uses many medical procedures where electricalsignals or currents are received from or delivered to a patient's body.The interface between medical equipment used in these procedures and theskin of the patient is usually some sort of biomedical electrode. Suchelectrodes typically include a conductor which must be connectedelectrically to the equipment, and a conductive medium adhered to orotherwise contacting skin of the patient, and they are of varying typeswith a wide variety of design configurations which will generally dependon their intended use and whether for example they are to be used astransmission electrodes or sensing i.e. monitoring electrodes.

[0004] Among the therapeutic procedures using biomedical electrodes aretranscutaneous electric nerve stimulation (TENS) devices used for painmanagement; neuromuscular stimulation (NMS) used for treating conditionssuch as scoliosis; defibrillation electrodes to dispense electricalenergy to a chest cavity of a mammalian patient to defibrillate heartbeats of the patient; and dispersive electrodes to receive electricalenergy dispensed into an incision made during electrosurgery.

[0005] Among diagnostic procedures using biomedical electrodes aremonitors of electrical output from body functions, such aselectrocardiograms (ECG) for monitoring heart activity and fordiagnosing heart abnormalities.

[0006] For each diagnostic, therapeutic, or electrosurgical procedure,at least one biomedical electrode having an ionically conductive mediumcontaining an electrolyte is adhered to or is otherwise contacted withmammalian skin at a location of interest and is also electricallyconnected to electrical diagnostic, therapeutic, or electrosurgicalequipment. A critical component of the biomedical electrode is theconductive medium which serves as the interface between the mammalianskin and the diagnostic, therapeutic, or electrosurgical equipment, andwhich is usually an ionically conductive medium.

[0007] Biomedical electrodes are used among other purposes to monitorand diagnose a patient's cardiovascular activity. Diagnostic electrodesare used to monitor the patient immediately and are only applied to thepatient for about five to ten minutes. Monitoring electrodes, however,are used on patients in intensive care for up to three dayscontinuously. In contrast, Holter electrodes are used to monitor apatient during strenuous and daily activities.

[0008] Although all of the biomedical electrodes just referred to areused to record cardiovascular activity, each electrode requires specificfeatures or characteristics to be successful. Thus, the diagnosticelectrode does not have to remain adhered to a patient for extensiveperiods but it does have to adhere to hairy, oily, dry and wet skineffectively for the five to ten minutes of use. The monitoring electrodehas to adhere for a longer period of time although the patient is oftenimmobile during the monitoring period. The Holter electrodes issusceptible to disruption from adhesion due to physical motion,perspiration, water, etc., and therefore requires the best adhesion andat the same time comfort and electrical performance.

[0009] In the biomedical electrodes known in the prior art the ionicallyconductive medium which serves as an interface, between the skin of amammalian patient and the electrical instrumentation, ranges fromconductive gels and creams to conductive pressure sensitive adhesives.However, while the conductive media can be in the form of pressuresensitive conductive adhesives, for monitoring or Holter biomedicalelectrode use such conductive adhesives are not generally adequate ontheir own to maintain adhesion to mammalian skin and additionalhypoallergenic and hydrophobic pressure sensitive adhesives may beemployed around the conductive medium to provide the required mammalianskin adhesion. U.S. Pat. No. 5,012,810 (Strand et al.) and U.S. Pat.Nos. 4,527,087, 4,539,996, 4,554,924 and 4,848,353 (all Engel) areexamples of documents that disclose biomedical electrodes which have ahydrophobic pressure sensitive adhesive surrounding the conductivemedium.

[0010] In general, a desirable skin electrode is one which maintainsgood electrical contact with the skin and is free of localised currenthot spots, i.e. exhibits uniform conductivity. For example, it has beenfound that a prior art electrode utilising karaya gum tends to creep inuse and flatten out, exposing skin to possible direct contact with thecurrent distribution member or lead wire. A desirable skin electrodeshould also usually have a low electrical impedance.

[0011] It is an object of this invention to provide hydrogel adhesivespossessing controlled and predictable adhesive properties which may bereadily varied to suit different uses and, in the case of medicalelectrodes or similar devices, different configurations or applications.It is also an object of the invention to provide such hydrogel adhesiveswhich in addition may possess superior electrical characteristics ascompared to those commonly associated with bioadhesive hydrogels.

SUMMARY OF THE INVENTION

[0012] The performance of hydrogels as adhesives is related to thesurface energetics of the adhesive and of the adherend (for examplemammalian skin) and to the viscoelastic response of the bulk adhesive.The requirement that the adhesive wets the adherend to maximise the workof adhesion is well known. This requirement is generally met when theadhesive has a similar or lower surface energy to the adherend. Theviscoelastic properties, in particular the elastic or storage modulus(G′) and the viscosity modulus (G″) are important. They are measured bydynamic mechanical testing at different rad/s. Their values at low rad/s(approximately 0.01 to 1 rad/s) and high rad/s (100 to 1000 rad/s) hasbeen related to the wetting/creep behaviour and peel/quick stickproperties respectively. The choice, assembly and processing of theingredients of the hydrogel adhesive are usually targetted at making amaterial with a balance of properties suitable for pressure sensitiveadhesive applications. A balance between the quantities and nature ofpolymer, plasticiser and the degree of crosslinking/entanglement has tobe achieved.

[0013] The main electrical property of interest is the impedance.Performance standards have been drawn up by the American Association ofMedical Instruments (AAMI). In sensing electrode applications theelectrodes, consisting of the hydrogel adhesive and a suitableconductive support, are placed in pairs, adhesive to adhesive contact.The conductive support frequently has a Ag/AgCl coating in contact withthe adhesive. The measured impedance is dependent on both the quality ofthe Ag/AgCl coating and the adhesive. In this configuration the adhesivemust contain chloride ions. The concentration of chloride ionsinfluences the impedance such that increasing the concentration canlower impedance. It would be anticipated that the activity of the ions(as opposed to the concentration) would be important in determiningimpedance, but in practice the determination of ion activity in thesesystems is not a trivial matter. It has been found that an importantrequirement in the control of impedance is the water content and itsrelated activity, and in general adhesives with higher water activityhave lower impedances.

[0014] When water is lost from the hydrogel both the adhesive andelectrical properties are likely to change deleteriously. Whilst thepresence of glycerol or other polyhydric alcohols in other reportedformulations has been quoted to provide humectant properties to thehydrogel, it has been found that the most important parameter topreventing water loss is the activity of the water within the hydrogelwhich in turn depends on the nature and proportions of the othercomponents and manner of processing.

[0015] Water activity in the hydrogel adhesive is primarily dependent onthe water content and the nature of the polymeric components and the wayin which they are processed. Water activity has been shown to have abetter correlation with the growth of bacteria and moulds than watercontent. It has been found that organisms struggle to row at wateractivities less than 0.8. Enzyme activity has also been reported todecrease significantly below activity of 0.8. Water activity has alsobeen found to influence the adhesivity of the hydrogel adhesive in thatat water activities above about 0.75, they become less adhesive. Abioadhesive composition having a suitable balance of the characteristicsdiscussed above has now surprisingly been found.

[0016] According to the invention there is provided a bioadhesivecomposition characterised in that it has:

[0017] (i) a water activity of from 0.4 to 0.9;

[0018] (ii) an elastic modulus at 1 rad/s of from 700 to 15,000 Pa;

[0019] (iii) an elastic modulus at 100 rad/s of from 2000 to 40,000 Pa;

[0020] (iv) a viscous modulus at 1 rad/s of from 400 to 14,000 Pa;

[0021] (v) a viscous modulus at 100 rad/s of from 1000 to 35,000 Pa;

[0022] wherein the viscous modulus is less than the elastic modulus inthe frequency range of from 1 to 100 rad/s. Preferably the impedance at500 MHz is less than 10 ohms, more preferably less than 5 ohm. When thecomposition includes chloride ions, the impedance at 10 Hz on Ag/AgClelectrodes is less than 1000 ohm, preferably less than 500 ohm.

[0023] Examination of the rheological properties of the compositionshave been successfully used to characterise and differentiate adhesivebehaviour. Typically the elastic modulus (G′) and the viscous modulus(G″) are measured over a range of 0.01-100 rad/s at a given temperature.For skin applications the appropriate temperature is 37° C. The moduliat low rad/s values relate to the initial bonding of the adhesive toskin and the higher to the changes in moduli values associated withde-bonding. Methods of measuring G′ and G″ are well known; for example aRheometric Scientific RS-5 rheometer could be used.

[0024] The water activity of the composition can be measured usingimpedance methods with devices such as the Rotronic AWVC (manufacturedby Rotronic). The activity of water may also be determined by placingthe composition in environments of controlled humidity and temperatureand measuring the changes in weight. The relative humidity (RH) at whichthe composition does not change weight corresponds to the activity ofwater in the gel (RH/100). The use of saturated salt solutions toprovide the appropriate environmental conditions is well known. Allcompositions directly exposed to relative humidities less than thatcorresponding to the activity of water will be thermodynamically allowedto lose water. Exposure to greater relative humidities and thecomposition will gain weight.

[0025] The impedance values at 10 Hz can be measured as follows.Silver/Silver chloride electrodes are assembled from the compositions byplacing 25 mm by 25 mm samples onto silver/silver chloride coatedplastic eyelets (product of Micron Medical Products and marketed asplastic eyelets 107). The impedances of the compositions are recorded bycontacting the electrodes face to face via the compositions andconnecting to an Xtratek ET-65A ECG electrode tester (product of Xtratekof Lenexa, Kans.). The impedance at 500 MHz can be measured using animpedance meter from a 10 cm by 5 cm section of gel 0.5 cm thick placedbetween two conducting aluminium plates.

[0026] The bioadhesive composition preferably comprises an aqueousplasticiser, a copolymer of a hydrophilic unsaturated water-solublefirst monomer and a hydrophilic unsaturated water-soluble second monomerand a cross-linking agent, the first monomer having a tendencypreferentially to enhance the bioadhesive properties of the composition.

[0027] Preferably the first monomer has a tendency also to enhance themechanical strength of the composition according to the invention and/orthe second monomer has a tendency preferentially to increase the wateractivity of the composition. More preferably the second monomer also hasa tendency preferentially to lower the electrical impedance and therebyenhance the electrical conductivity of the composition.

[0028] The bioadhesive composition is preferably obtainable bypolymerising an aqueous reactive mixture comprising the said firstmonomer, the said second monomer and a crosslinking agent.

[0029] According to the invention there is further provided a biomedicalelectrode which comprises a bioadhesive composition according to theinvention in association with an electrically conductive interface. Thebiomedical electrode optionally further comprises a support. Theelectrically conductive interface preferably comprises a layer ofelectrically conductive material which is preferably applied to thesupport, when present.

[0030] The invention also provides a fixation product suitable forattaching a biomedical device to skin (or the human body) e.g. acatheter, tubing, wires or cables which product comprises a bioadhesivecomposition according to the invention.

[0031] In preferred embodiments the first and second monomers will beacrylate based monomers selected for their ability to polymerise rapidlyin water and having substantially the same molecular weight whereby in amixture, of the two the relative proportions may be varied withoutsignificantly altering the molar characteristics of the composition.

[0032] The first monomer is preferably a compound of formula

[0033] wherein R1 is an optionally substituted hydrocarbon moiety, R² ishydrogen or optionally substituted methyl and ethyl, and M representshydrogen or a cation.

[0034] R¹ is preferably an optionally substituted alkyl, cycloalkyl oraromatic moiety. Preferably R¹ represents a saturated moiety or anaromatic moiety. R¹ preferably contains from 3 to 12 carbon atoms, morepreferably from 3 to 6 carbon atoms. A preferred moiety which R¹represents is

[0035] wherein R³ represents hydrogen or an optionally substitutedstraight or branched chain alkyl group possessing from 1 to 6 carbonatoms and R⁴ represents an optionally substituted straight or branchedchain alkyl group possessing from 1 to 6 carbon atoms.

[0036] The second monomer is preferably a compound of formula

[0037] wherein R⁵ represents hydrogen or optionally substituted methylor ethyl, R⁶ represents hydrogen or a cation and R⁷ represents anoptionally substituted alkyl moiety of 1 to 4 carbon atoms. PreferablyR⁷ represents optionally substituted n-propyl.

[0038] R¹, R², R³, R⁴, R⁵ and R⁷ are optionally substituted by a groupwhich preferably has a tendency to increase the water solubility of thecompound. Suitable groups will be well known to a person of skill in theart. A preferred optional substituent is a hydroxyl, amino or ammoniumgroup or a halogen (e.g. chlorine, bromine, or iodine) atom. A suitablecation is an alkali metal cation, especially sodium or potassium.

[0039] Most preferably the first monomer is2-acrylamido-2-methylpropanesulphonic acid or an analogue thereof or oneof its salts, e.g. an alkali metal salt such as a sodium, potassium orlithium salt, while the second monomer is a polymerisable sulphonate ora salt, e.g. an alkali metal salt such as a sodium, potassium or lithiumsalt, of acrylic acid (3-sulphopropyl)ester or an analogue thereof.Particular preferred examples of these respective monomers are thesodium salt of 2-acrylamido-2-methylpropanesulphonic acid, commonlyknown as NaAMPS, and acrylic acid (3-sulphopropyl)ester potassium salt,commonly known as SPA. NaAMPS is available commercially at present fromLubrizol as either a 50% aqueous solution (reference code LZ2405) or a58% aqueous solution (reference code LZ2405A). SPA is availablecommercially in the form of a solid from Raschig.

[0040] The total monomer content in the aqueous reactive mixture ispreferably from 15% to 60% by weight, preferably from 20% to 50% byweight.

[0041] In preferred embodiments the ratio by weight of the first monomerto the second monomer is from 20:1 to 2:3, preferably 10:1 to 2:3; morepreferably in the range 60:40 to 40:60, and may sometimes beapproximately 50:50.

[0042] The first monomer is preferably included in an amount by weightof from 1% to 60%, more preferably from 5% to 50%, most preferably from15% to 40%. The second monomer is preferably included in an amount byweight of from 1% to 50%, preferably from 10% to 30%, most preferablyfrom 10% to 20%. The crosslinker is preferably included in an amount offrom 0.01% to 2%, more preferably from 0.1 to 2% by weight. The balanceof the composition preferably comprises an aqueous plasticiser.

[0043] One advantage of the first and second monomers is that it hasbeen found that high monomer content solutions can be achieved(approximately 75%). It has also been found that the second monomer issoluble in polyhydric alcohols such as glycerol, and addition ofglycerol to the first and second monomer mixture enhances thesolubilisation process. It has been found that the combination of thetwo monomers enables a greater control over water content than can beachieved otherwise. This can be important because it has also been foundthat compositions made with the final water content as an integral partof the pre-gel mix have different properties from those made with anexcess of water and then dried to the final composition. For example,hydrogels with a final composition obtained by the evaporation of watergenerally have lower elastic or storage moduli than those made with noevaporation of water. To obtain similar levels of elastic moduli, theamount of crosslinker required in the former materials is higher. Theevaporation of water and extra crosslinker add to the cost of theprocess. This problem is avoided by the present invention where a finaldrying step is generally not required.

[0044] Conventional crosslinking agents are used to provide thenecessary mechanical stability and to control the adhesive properties ofthe composition. Although compositions can be made with suitableadhesive and electrical properties, a sufficient amount of a suitablecross-linker must be used; if too little crosslinker is used, convertingthe material into a completed electrode becomes impossible. Typicalcrosslinkers include tripropylene glycol diacrylate, ethylene glycoldimethacrylate, alkoxylated triacrylate, polyethylene glycol diacrylate(PEG400 or PEG600), methylene bis acrylamide.

[0045] The aqueous reactive mixture optionally further comprises asurfactant, an additional monomer, an electrolyte, a processing aid(which is preferably a hydrophobic polymer), a water soluble polymersuitable for forming an interpenetrating polymer network, anon-hydrophilic polymer, an antimicrobial agent (e.g. citric acid,stannous chloride) and/or, for drug delivery applications,pharmaceutically active agents, the latter being designed to bedelivered either passively (e.g. transdermally) or actively (e.g.iontophoretically) through the skin.

[0046] The process used to prepare bioadhesive compositions inaccordance with the invention comprises mixing the ingredients toprovide a reaction mixture in the form of an initial pre-gel aqueousbased liquid formulation, which is then converted into a gel by a freeradical polymerisation reaction. This may be achieved for example usingconventional thermal initiators and/or photoinitiators or by ionizingradiation. Photoinitiation is a preferred method and will usually beapplied by subjecting the pre-gel reaction mixture containing anappropriate photoinitiation agent to UV light after it has been spreador coated as a layer an siliconised release paper or other solidsubstrate. The processing will generally be carried out in a controlledmanner involving a precise predetermined sequence of mixing and thermaltreatment or history. One preferred feature of the process according tothe invention is that no water is removed from the hydrogel aftermanufacture.

[0047] Additional Monomer

[0048] The composition according to the invention preferably comprisesone or more additional monomers. A suitable additional monomer is anon-ionic monomer or ionic monomer. If the monomer is ionic, it iseither anionic or cationic. Additional monomers, when present, arepreferably included in an amount of up to 10% by weight.

[0049] A preferred non-ionic monomer is a N-disubstituted acrylamide(preferably an N,N-dialkylacrylamide) or an analogue thereof.N,N-dimethylacrylamide (NNDMA) and/or an analogue thereof isparticularly preferred.

[0050] A preferred cationic monomer is a quaternary ammonium salt. Anespecially preferred cationic monomer is (3-acrylamidopropyl)trimethylammonium chloride or [2-(acryloyloxy)ethyl]trimethyl ammonium chloride.

[0051] A preferred anionic monomer is an acrylate based monomer such asacrylic acid or a salt or ester thereof.

[0052] Plasticiser

[0053] The compositions according to the invention generally comprise,in addition to a crosslinked polymeric network, an aqueous plasticisingmedium and, optionally, additional electrolyte. Plasticisers aregenerally used in the invention to control adhesive properties.

[0054] The aqueous plasticising medium optionally additionally comprisesa polymeric or non-polymeric polyhydric alcohol (such as glycerol), anester derived therefrom and/or a polymeric alcohol (such as polyethyleneoxide). Glycerol is the preferred plasticiser. An alternative preferredplasticiser is an ester derived from boric acid and a polyhydric alcohol(such as glycerol). The aqueous reactive mixture preferably comprisesfrom 10% to 50%, preferably from 10% to 45%, of plasticiser (other thanwater) by weight of the mixture.

[0055] It is well known that water in hydrogels can be present in atleast two forms, freezing and non-freezing, as measured by DifferentialScanning Calorimetry. In many examples of commercially availablehydrogels the water is present only as non freezing water. It has beenfound, however, that compositions with useful adhesive propertiescomprising the first and second monomers can be made which have bothfreezing and non-freezing water, and the water activity in such gels isgenerally high. One advantage of including the second monomer is that ithas a tendency to increase the likelihood that the compositions willcontain freezing water. The advantage gained by the presence of freezingwater becomes evident in the application of these gels to stressmonitoring ECG. In certain cases the preferred medium for interfacingthe monitoring instrument with the body is a “wet gel”. It has beensuggested that the advantage gained by “wet gels” is in the wetting ofthe skin and consequent lowering of skin impedance, but it has beenfound in clinical trials that hydrogels with freezing water can matchthe performance of “wet gels”.

[0056] Electrolyte

[0057] When the compositions are intended for use in conjunction withAg/AgCl medical electrodes, chloride ions are required to be present inorder for the electrode to function. Accordingly the compositionspreferably include an electrolyte except where the composition comprisesan additional monomer which is a cationic monomer in the form of achloride salt. Potassium chloride and sodium chloride are commonly used.However, any compound capable of donating chloride ions to the systemmay be used, for example lithium chloride, calcium chloride, ammoniumchloride. The amount that should be added is dependent on the electricalproperties required and is typically from 0.2 to 7% by weight. Indesigning the compositions for lowest impedance as measured under theAAMI standard, allowance must be given for the amount and activity ofwater. These factors will control the effective ion activity and hencethe amount of chloride available for participating in theelectrochemistry of the system. Compositions with lower chlorideconcentration but higher water activity have lower impedances.

[0058] Interpenetrants

[0059] The compositions preferably additionally comprise a water solublepolymer suitable for forming an interpenetrating polymer network.Hydrogels based on interpenetrating polymer networks (IPN) are wellknown. An IPN has been defined as a combination of two polymers, each innetwork form, at least one of which has been synthesised and/orcrosslinked in the presence of the other. As will be appreciated, thiscombination will generally be a physical combination rather than achemical combination of the two polymers. IPN systems may be describedby way of example as follows:

[0060] Monomer 1 is polymerised and crosslinked to give a polymer whichis then swollen with monomer 2 plus its own crosslinker and initiator.

[0061] If only one polymer in the system is crosslinked the networkformed is called a semi-IPN. Although they are also known as IPN's, itis only if there is total mutual solubility that full interpenetrationoccurs. In most IPN's there is, therefore, some phase separation butthis may be reduced by chain entanglement between the polymers. It hasalso been reported that semi IPN's can be made in the presence ofcarrier solvents (for example water in the case of hydrophiliccomponents).

[0062] It has been found that polymerising and crosslinking watersoluble monomers in the presence of water soluble polymers, water andpolyhydric alcohols produces hydrogel materials with enhancedrheological and consequently adhesive properties.

[0063] Suitable water soluble polymers for the formation of semi IPN'sinclude poly (2-acrylamido-2-methylpropanesulphonic acid) or one of itssalts and its copolymers, poly (acrylic acid-(3-sulphopropyl)esterpotassium salt), copolymers of NaAMPS and SPA, polyacrylic acid,polymethacrylic acid, polyethylene oxide, polyvinyl methyl ether,polyvinyl alcohol, polyvinyl-pyrrolidone, its copolymers with vinylacetate, dimethylaminoethyl methacrylate, terpolymers withdimethylaminoethyl methacrylate and vinyl-caprolactam, polysaccharidessuch as gum arabic, karaya gum, xanthan gum, guar gum, carboxymethylcellulose (CMC), NaCMC, hydroxypropylmethyl cellulose (HPMC),hydroxyethyl cellulose (HEC) or combinations thereof.

[0064] The amount of interpenetrant polymer used will be dependent onthe mechanical and rheological properties required as well onconsideration of processing conditions. If the interpenetrant polymerused increases the viscosity of the pre-gel mix beyond 5000 centipoiseit has been found that the monomers do not polymerise and crosslink onan acceptable time scale (should be less than 60 seconds, preferablyless than 10 seconds). The viscosity depends on the nature and molecularweight of the interpenetrant and the nature of pre-gel processing.

[0065] Of the natural polysaccharides, gum arabic or maltodextrin isusually preferred due to its cold water solubility and lesser effect onviscosity compared with, for example, karaya gum. A higher concentrationof gum arabic than karaya may therefore be used if desired, enabling awider control of hydrogel properties. It has also been found that theprocessing steps for assembling the pre-gel formulation can be criticalwith respect to the properties of the manufactured hydrogel. For a givenformulation, if the components are assembled at 25° C. and cureddifferent electrical and adhesive properties are obtained compared tothose that have been heated to 70° C. Whilst adhesive properties may beenhanced, electrical properties e.g. low frequency impedance, can bedowngraded. Solutions containing natural polysaccharides become lessopaque indicative of improved solubility. The activity of water incompositions prepared from heat treated pre-gels generally is lower thanin non heat treated pre-gels.

[0066] Other Additives

[0067] The composition preferably comprises a hydrophobic polymer.Hydrophobic polymers may be incorporated either in the presence orabsence of interpenetrant polymers to form phase separated materials.The preparation of two phase composites consisting of a hydrophilicpolymer containing an ionically conducting continuous phase and domainsof a hydrophobic pressure sensitive adhesive which enhance adhesion tomammalian skin have been reported in U.S. Pat. No. 5,338,490. The methodof preparation described therein involved casting a mixture (as asolution and or suspension) consisting of the hydrophilic polymercontaining phase and hydrophobic components onto a substrate and thenremoving the solvent. It has been found, however, that adhesiveionically conducting hydrogels may be better prepared by combining thehydrophobic polymer (preferably as an emulsion) with the components ofthe pre-gel reaction mixture and casting these onto a substrate andcuring. In other words, there is no need to remove a solvent in order toform useful materials. Furthermore, the hydrophilic phase of thecomposition in addition to being a crosslinked network may also be anIPN or semi IPN.

[0068] It is believed that when hydrophobic polymers are incorporated inthis way that the hydrophobic component segregates to the surface (asdetermined by Fourier transform infrared attenuated total reflectancespectroscopy, FTIR ATR, approximate sampling depth 1 μm using a ZnSecrystal or 0.25 μm with a Germanium crystal) and that it is the amountof the hydrophobic component present in the surface that influences theadhesion to a wide variety of materials. The greater the amount of thehydrophobic component in the surface the greater the adhesion. In U.S.Pat. No. 5,338,490 weight ratios of the hydrophilic phase to thehydrophobic phase of 60:1 to 8:1 were claimed. In hydrogel adhesives ofbetween 100 to 2000 microns thick made in accordance with the presentinvention, ratios of hydrophilic to hydrophobic components ranging from7:1 to 1:20 have been found to be preferable, especially when theseratios are present in the surface of the adhesive composition. In theprocess of the present invention, however, it may take up to 72 hoursfrom the initial curing of the adhesive hydrogel for the segregation ofthe hydrophobic materials to the surface, as defined by the ATR samplingdepth, to be complete.

[0069] Preferably, the hydrophobic pressure sensitive adhesive in suchembodiments is selected from the group consisting of polyacrylates,polyolefins, silicone adhesives, natural or synthetically derived rubberbase and polyvinyl ethers or blends thereof. Preferably the hydrophobicpressure sensitive adhesive in these embodiments is an ethylene/vinylacetate copolymer such as that designated DM137 available from HarlowChemicals or vinyl acetate dioctyl maleate such as that designatedFlexbond 150 and sold by Air Products. Those skilled in the art willalso know that the molecular weight and comonomer ratios may be alteredto control the properties of hydrophobic pressure sensitive adhesives.In general, the degree of surface segregation exhibited by suchhydrophobic pressure sensitive adhesive (HPSA) will be dependent onfactors such as composition of the HPSA, viscosity of the pre-gelmixture, temperature and rate of curing.

[0070] The bioadhesive composition according to the invention preferablyis such that the relative amount of hydrophobic polymer (which is theamount of hydrophobic polymer relative to the amount of monomer) ispreferably at least four times greater, more preferably at least eighttimes greater, at the surface of the composition compared to what it isin the bulk of the composition. The relative amount at the surface ispreferably the relative amount in the composition at a depth of up to 1micron (as measured using FTIR ATR using a ZnSe crystal), preferably upto 0.25 micron (as measured using FTIR ATR using a Germanium crystal).The relative amount is measured by obtaining the ratio of the peakheight of the peak in the carbonyl region for the hydrophobic polymer tothe peak height of the peak in the carbonyl region for the firstmonomer, using the relevant FTIR ATR technique. The wave number valuesfor the relevant peaks for the hydrophobic polymer and the monomer arewell known.

[0071] More preferably, the ratio of the relative amount in the surfaceof the composition at a depth of up 0.25 micron to the relative amountin the surface of the composition at a depth of up 1 micron is more than1:1, more preferably more than 1.25:1, most preferably more than 1.5:1.

[0072] Surfactant

[0073] The composition according to the invention optionally includes asurfactant.

[0074] Any compatible surfactant may be used. Nonionic, anionic andcationic surfactants are preferred, either alone or in combination. Thesurfactant is preferably included in an amount from 0.1% to 20% byweight, more preferably 0.1% to 10% by weight.

[0075] The bioadhesive compositions according to the invention are alsouseful in a variety of consumer care applications. For example they canbe used as the adhesive for a faecal management device, wound dressingor prosthesis, e.g. hair prosthesis.

[0076] The addition of citric acid is also of interest since it also hasthe capacity to decrease the electrical impedance as hereinafterdescribed in connection with EXAMPLE 4.

[0077] The invention will be further described with reference to thegraphs of FIGS. 1 to 5 of the accompanying drawings and the followingExamples in connection with bioadhesive compositions suitable for use inmedical skin electrodes or in fixation products.

EXAMPLE 1

[0078] In 20 parts of polyethylene glycol diacrylate (pEG600) (productof UCB Chemicals marketed under the trade name designation of Ebacryl11) were dissolved 6 parts of 1-hydroxycyclohexyl phenyl ketone (productof Ciba and marketed under the trade name designation of Irgacure 184).The solution so produced is herein designated solution A (XL/PI).Separately, 50 parts of the potassium salt of 3-sulphopropyl acrylate(SPA) (product of Raschig) were dissolved in 50 parts water to formsolution B. A further solution designated solution C consisted of 50parts water, 50 parts of the sodium salt of 2-acrylamido-2-methylpropanesulphonic acid (NaAMPS) product of the Lubrizol Corporation and marketedas a 50% aqueous solution under the trade name LZ2405). Mixtures ofsolutions B and C in the ratios of 100:0, 90:10, 60:40, 50:50, 40:60,10:90 and 0:100 were made to form pre-gel solutions. To 80 parts of eachof these pre-gel solutions, 0.15 parts of solution A, 5 parts potassiumchloride and 20 parts distilled water were added. The pre-gel solutionswere coated onto siliconised release paper at a coat weight of 0.8kilograms per square meter and exposed to ultraviolet radiation by beingpassed under a medium pressure mercury arc lamp at a speed of 5 metersper minute to form clear self supporting gels. The residence time underthe lamp was 4 seconds. The storage moduli(G′) of 20 mm diameter discsstamped from the gels were recorded on a Rheometric Scientific RS-5rheometer at 37° C. The G′ values at 1 rad are recorded in Table 1.Silver/Silver chloride electrodes were assembled from the gels byplacing 25 mm by 25 mm samples onto silver/silver chloride coatedplastic eyelets (product of Micron Medical Products and marketed asplastic eyelets 107). The impedances of the gels were recorded bycontacting the gelled electrodes face to face via the gels andconnecting to an Xtratek ET-65A ECG electrode tester (product of Xtratekof Lenexa, Kans.). The impedance data are recorded in Table 1. Thepercentage of freezing water present in the gels, also recorded in Table1, were obtained by cooling small weighed samples of gel (ca. 2 mg) insealed aluminium pans to −70° C. and heating at 10° C. per minute in aPerkin Elmer differential scanning calorimeter, DSC2. Using acalibration graph produced from samples of pure water, the area underthe observed endotherm peaks was converted to the weight of freezingwater in the sample. With the exception of the gels containing 90 and100 parts SPA, the gels produced had acceptable tack and peel propertieson the skin. From the data in Table 1 relatively linear changes instorage modulus and freezing water content are obtained on increasing ordecreasing the SPA to NaAMPS ratio. The changes in impedance are smallbut surprisingly appear to be non linear.

[0079] In the above Example, and in the following Examples whereverparts are mentioned they are meant as parts by weight unless otherwisespecified. TABLE 1 NaAMPS 80 72 48 40 32 8 0 SolutionC SPA 0 8 32 40 4872 80 SolutionB Distilled 20 20 20 20 20 20 20 Water XL/PI 0.15 0.150.15 0.15 0.15 0.15 0.15 SolutionA KCl 5 5 5 5 5 5 5 G′(Pa) @ 4,1983,389 2,471 2,205 1,759 703 492 1 rad/s Impedance 44 43 41 41 39 38 38(Ohms) % 20 24 30 34 35 41 44 Freezing Water

EXAMPLE 2

[0080] In 20 parts of polyethylene glycol diacrylate (pEG600) (productof UCB Chemicals marketed under the trade name designation of Ebacryl11) 6 parts of 1-hydroxycyclohexyl phenyl ketone (product of Ciba andmarketed under the trade name designation of Irgacure 184) weredissolved. (This solution is designated solution A) (XL/PI). Separately58 parts of the potassium salt of 3-sulphoproylacrylate (SPA) (productof Raschig) were dissolved in 58 parts distilled water to form solutionD. A further solution designated solution E consisted of 42 parts water,58 parts of the sodium salt of 2-acrylamido-2-methylpropane sulphonicacid (NaAMPS) (a product of the Lubrizol Corporation marketed as a 58%aqueous solution under the trade name LZ2405A). Mixtures of solutions Dand E in the ratios 100:0, 90:10, 60:40, 50:50, 40:60, 10:90 and 0:100were made to form pre-gel solutions. To 100 parts of each of thesepre-gel solutions, 0.17 parts of solution A and 3 parts potassiumchloride were added. The pre-gel solutions were coated onto siliconisedrelease paper at a coat weight of 0.8 kilograms per square meter andpassed under a medium pressure mercury arc lamp at a speed of 5 metersper minute to form clear self-supporting gels. Storage moduli,impedances and % freezing water were measured as in Example 1 and arerecorded in Table 2. As in the gels described in Example 1 the changesin the elastic or storage modulus G′(Pa) are linear with respect to theincreasing or decreasing ratio of NaAMPS to SPA. However, surprisinglyboth the impedance and % freezing water content exhibit distinctnon-linear behaviour. All the gels produced possess acceptable tack andpeel strength against skin. The gels with NaAMPS:SPA ratios in the rangeof 60:40 to 40:60, however, have a better balance of reusability andpeel strength. TABLE 2 NaAMPS 100 90 60 50 40 10 0 SolutionE SPA 0 10 4050 60 90 100 SolutionD XL/PI 0.17 0.17 0.17 0.17 0.17 0.17 0.17SolutionA KCl 3 3 3 3 3 3 3 G′(Pa) @ 15,142 14,333 11,073 10,672 9,9206,280 5,199 1 rad/s Impedance 62 61 49 46 43 40 40 (Ohms) % 0 0 0.5 11.8 23 25 Freezing Water

[0081] Upon varying the amount of the cross-linking agent asubstantially linear change in the elastic modulus G′ can also beobtained, as illustrated by the graph of FIG. 1.

EXAMPLE 3

[0082] To 57 parts of a 58% solution of the sodium salt of2-acrylamido-2-methylpropane sulphonic acid (NaAMPS) (LZ2405A) 10 partsof a 58% solution of the potassium salt of 3-sulphopropyl acrylate (SPA)were added along with 5 parts potassium chloride and stirred until thepotassium chloride has dissolved. This solution was then mixed with 30parts glycerol for 30 minutes. To the latter solution were added 0.15parts of a solution containing 20 parts of polyethylene glycoldiacrylate (pEG600) (product of UCB Chemicals marketed under the tradename designation of Ebacryl 11) in which 6 parts of 1-hydroxycyclohexylphenyl ketone (product of Ciba and marketed under the trade namedesignation of Irgacure 184) were dissolved. The so-formed pre-gelsolution was then cured as in Example 1. The impedance of the resultinggel, measured as described in Example 1, was 83 Ohms. Good skin adhesionproperties were obtained for this gel. The impedance of a similar gelmade from 67 parts of a 58% solution of the sodium salt of2-acrylamido-2-methylpropane sulphonic acid but with no SPA had animpedance of 105 Ohms. This demonstrates that the presence of SPA givesrise to a reduction in impedance.

EXAMPLE 4

[0083] The method of Example 3 was repeated with 1 part citric acidbeing added with the potassium chloride. The impedance of a similar gel(denoted 4B in Table 3) made from 67 parts of a 58% solution of thesodium salt of 2-acrylamido-2-methylpropane sulphonic acid but no SPAhad an impedance of 96 Ohms demonstrating again that the addition of SPAreduces the impedance. From the data summarised in TABLE 3, however, itis seen that addition of citric acid also gives rise to a reduction inimpedance, and the effect is surprisingly large for a gel with SPA andNaAMPS. The adhesion to skin and reusability characteristics for thisgel of Example 4 containing citric acid and SPA were better than the geldescribed in Example 3. TABLE 3 Example 3A 3B 4A 4B NaAMPS 67 57 67 57(58% soln) SPA 0 10 0 10 (58% soln) Glycerol 30 30 30 30 Citric Acid 0 01 1 Crosslinker/ 0.15 0.15 0.15 0.15 Photoinitiator Impedance 105 85 9661 (ohms)

EXAMPLE 5

[0084] The formulations listed in Table 4 were prepared using thefollowing method which is for formulation 5a. To 58 parts of a 50%aqueous solution of the sodium salt of 2-acrylamido-2-methylpropanesulphonic acid (NaAMPS) (LZ2405) 2 parts of the potassium salt of3-sulphopropyl acrylate (SPA) were added along with 1.575 parts ofacrylic acid and stirred. This solution was then mixed with 37 partsglycerol for 30 minutes. To the latter solution were added 0.175 partsof solution (F). Solution F contains 20 parts of an alkoxylatedtriacrylate (product of UCB Chemicals marketed under the trade namedesignation of IRR 210) in which 1.4 parts of 1-hydroxycyclohexyl phenylketone (product of Ciba and marketed under the trade name designation ofIrgacure 184) are dissolved. The so-formed pre-gel solution was thencured as in Example 1. The G′ and G″ moduli were measured from 20 mmdiameter discs of the gel using a Rheometric Scientific RS-5 rheometerat 37° C.

[0085] To prepare formulation 5b, the same method was repeated exceptthat 0.15 parts of solution F were used instead of 0.175 parts.

[0086] To prepare formulations 5c and 5d, the same method used forformulation 5a was repeated except that the parts by weight were changedto the figures given in Table 4A. The potassium chloride was addedinstead of the acrylic acid; for formulation 5d, deionised water wasalso added. TABLE 4 Composition in parts by weight Formulation 5a 5b 5c5d 50% NaAMPS 58 58 75 75 KCl 5 5 Acrylic Acid 1.575 1.575 SPA 2 2 2 2Glycerol 37 37 25 25 DI WATER 3 PI/XL (Solution) 0.175 (F) 0.15 (F) 0.15(A) 0.15 (A) G′(Pa), @ 1 rad/s 1455 1054 G′Pa) @ 100 5174 4613 rad/sG″(Pa) @ 1 601 488 rad/s G″(Pa) @ 100 2906 2640 rad/s

EXAMPLE 6

[0087] The formulations listed in Table 5 were prepared using thefollowing method which is for formulation 6a. To 67 parts of a 58%aqueous solution of the sodium salt of 2-acrylamido-2-methylpropanesulphonic acid (NaAMPS) (LZ2405A) 2 parts of the potassium salt of3-sulphopropyl acrylate (SPA) were added along with 5 parts of potassiumchloride and 1 part of citric acid and stirred until the potassiumchloride had dissolved. This solution was then mixed with 30 partsglycerol for 30 minutes. To the latter solution were added 0.13 parts ofsolution A prepared as described in Example 1. The so-formed pre-gelsolution was then cured as in Example 1. The G′ and G″ moduli weremeasured from 20 mm diameter discs of the gel using a RheometricScientific RS-5 rheometer at 37° C.

[0088] To prepare formulation 6b, the same method was repeated exceptthat the potassium chloride and citric acid were omitted, 0.06 parts byweight of solution G were used instead of solution A and the amounts ofthe other ingredients were changed to the amounts given in Table 5.Solution G contains 20 parts of polyethylene glycol diacrylate(molecular weight 400) (product of UCB Chemicals marketed under thetrade name designation of IRR 280) in which 6 parts of1-hydroxycyclohexyl phenyl ketone (product of Ciba and marketed underthe trade name designation of Irgacure 184) are dissolved.

[0089] To prepare formulations 6c and 6d, the same method used forformulation 6a was repeated except that citric acid was omitted, 0.06parts of solution G were used instead of solution A and the parts byweight were changed to the figures given in Table 5.

[0090] To prepare formulation 6e, the same method used for formulation6a was repeated except that gum arabic and the ethylene/vinyl acetatecopolymer designated DM137 and sold by Harlow Chemicals were addedinstead of citric acid and the parts by weight were changed to thefigures given in Table 5.

[0091] To prepare formulation 6f, the same method used for formulation6a was repeated except that the ethylene/vinyl acetate copolymerdesignated DM137 and sold by Harlow Chemicals, polyethylene glycol(molecular weight 400) and sodium nitrate were added with the citricacid and the parts by weight were changed to the figures given in Table5. TABLE 5 Composition in parts by weight Formulation 6a 6b 6c 6d 6e 6f58% NaAMPS 67 57 57 57 67 50 KCl 5 5 5 5 1 Citric Acid 1 1 SPA 2 10 1010 2 18 Glycerol 30 33 33 28 30 20 Gum Arabic 2 DM 137 2 3 PEG 400 10Sodium Nitrate 0.05 PI/XL (Solution) 0.13 (A) 0.06 0.06 (G) 0.075 0.25(A) 0.175 (G) (G) (A) G′(Pa) @ 1 2973 4326 3019 4637 rad/s G′(Pa) @ 1009800 13986 9763 8789 rad/s G″(Pa) @ 1 1265 1914 1200 1029 rad/s G″(Pa) @100 4597 6707 4537 3952 rad/s

EXAMPLE 7

[0092] To 34.7 parts of a 58% aqueous solution of the sodium salt of2-acrylamido-2-methylpropane sulphonic acid (NaAMPS) (LZ2405A) 34.7parts of a 58% aqueous solution of the potassium salt of 3-sulphoproylacrylate (SPA) were added along with 4.6 parts potassium chloride and 3parts distilled water and stirred until the potassium chloride hasdissolved. This solution was then mixed with 23.2 parts glycerol for 30minutes. To the latter solution were added 0.15 parts of solution Aprepared as described in Example 1. The so-formed pre-gel solution wasthen cured as in Example 1. The impedance of the resulting gel asmeasured as described in Example 1 was 48 Ohms. In vivo tests on thethigh skin of a Caucasian male using a frequency response analyser(Solartron 1172) and skin impedance analyser in a three electrodeconfiguration (test, control and reference) indicated that this gel hasthe electrical characteristics of commercially available wet gelsdespite having the mechanical characteristic associated with mostcommercially available hydrogel adhesives.

EXAMPLE 8

[0093] To 20 parts glycerol, 3 parts of a hydrophobic ethylene/vinylacetate copolymer emulsion (50% solids) (product of Harlow Chemicalsmarketed under the trade name DM137) and 10 parts polyethylene glycol(molecular weight 600) were added and stirred until a uniform colour wasobtained. To this mixture were added 50 parts of a 58% solution of thesodium salt of 2-acrylamido-2-methylpropane sulphonic acid (NaAMPS)(LZ2405A), 16 parts potassium salt of 3-sulphopropyl acrylate (SPA) and5 parts potassium chloride, and the solution was heated with stirring to60° C. for one hour. The mixture had changed from an opaque off white toa translucent off white appearance. The turbidity of the solutions asmeasured in a portable turbidity meter, product code H193703 marketed byHanna had changed from 254 ftu to 107 ftu. The solution was cooled to20° C. and then there was added 0.13 parts of solution A prepared asdescribed in Example 1. This final solution was stirred for one hour andthen cured as in Example 1. The resulting gel had an impedance of 254Ohms and a G′ value at 1 rad of 5328 Pa. The activity of water in thegel, as determined by placing the gel into cabinets at varying levels ofhumidity at 40° C. (40, 52, 64 and 80% ORH) and measuring weight uptakeor loss and extrapolating to zero weight change, was 0.62. The adhesionto skin of this gel was significantly greater than those described inthe previous examples. Analysis of the gel by attenuated totalreflectance infra-red spectroscopy revealed that in the surface regions(about 1 micron or less), either the air surface or the surface incontact with the release paper, the concentration of the ethylene/vinylacetate copolymer relative to the NaAMPS was significantly enhancedcompared to the bulk composition.

EXAMPLE 9

[0094] The method of Example 8 was carried out except that with theglycerol were added 3 parts of gum arabic. The resulting gel had animpedance of 358 Ohms and a G′ value at 1 rad of 5406 Pa. The activityof water as determined by the method in Example 8 was 0.55. The adhesionto skin of this gel was significantly greater than those described inthe previous examples. Analysis of the gel by attenuated totalreflectance infra-red spectroscopy revealed that in the surface region(about 1 micron or less), either the air surface or the surface incontact with the release paper, the concentration of the ethylene/vinylacetate copolymer relative to the NaAMPS was significantly enhancedcompared to the bulk composition.

EXAMPLE 10

[0095] The formulations shown in Tables 6 and 7 were prepared using thefollowing method which is for formulation 10a. To 20 parts glycerol, 15parts of a hydrophobic vinyl acetate/dioctyl maleate copolymer emulsion(product of Air Products marketed under the trade name Flexbond 150)were added and stirred until a uniform colour was obtained. To thismixture were added 44 parts of a 58% solution of the sodium salt of2-acrylamido-2-methylpropane sulphonic acid (NaAMPS) (LZ2405A), 20 partspotassium salt of 3-sulphopropyl acrylate (SPA) and 4 parts potassiumchloride, and the solution was heated with stirring to 60° C. for onehour. The solution was cooled to 20° C. and then there was added 0.13parts of solution G prepared as described in Example 6. This finalsolution was stirred for one hour and then cured as in Example 1. The G′and G″ moduli were measured from 20 mm diameter discs of the gel using aRheometric Scientific RS-5 rheometer at 37° C.

[0096] Fourier transform infrared attenuated total reflectance spectra(FTIR ATR) were taken of both the pregel mixture and of the gel formedafter polymerisation using a ZnSe crystal (approximate sampling depth 1μm). The results obtained are shown in FIGS. 2 and 3, respectively. Thepeak at around 1740 cm⁻¹ corresponds to the hydrophobic polymer whereasthe peak at around 1550 cm⁻¹ corresponds to NaAMPS. It can be seen thatbefore polymerisation the ratio in height of the former peak to thelatter peak is about 0.25:1 whereas after polymerisation, the ratio isabout 2.9:1. This shows a twelve-fold increase in the concentration ofthe hydrophobic polymer at the surface of the gel after polymerisationindicating that the hydrophobic polymer surface segregates. A furtherFTIR ATR spectrum was taken of the gel formed after polymerisation usinga germanium crystal (approximate sampling depth 0.25 μm). It was foundthat the ratio in the height of the former peak to the latter peak is3.9:1 showing a sixteen fold increase in the concentration of thehydrophobic polymer on the surface of the gel.

[0097] To prepare formulation 10b, the same method used for formulation10a was repeated except that a hydrophobic ethylene/vinyl acetatecopolymer emulsion (50% solids) (product of Harlow Chemicals marketedunder the trade name DM137) was used instead of Flexbond 150, 3 partspolyethylene glycol (molecular weight 600) were added with thehydrophobic copolymer DM137 and the parts by weight were changed to thefigures given in Table 6.

[0098] FTIR ATR were taken of the gel formed after polymerisation usinga ZnSe crystal (approximate sampling depth 1 μm) and a germanium crystal(approximate sampling depth 0.25 μm). The results obtained are shown inFIGS. 4 and 5, respectively. As for formulation 10a, the peak at around1740 cm⁻¹ corresponds to the hydrophobic polymer whereas the peak ataround 1550 cm⁻¹ corresponds to NaAMPS. The ratio of the former peak tothe latter peak for FIG. 4 (the ZnSe FTIR ATR spectrum) is about 21:1whereas the ratio for FIG. 5 (the germanium FTIR ATR spectrum) is about11:1. This again demonstrates the hydrophobic polymer segregates to thesurface of the gel.

[0099] To prepare formulation 10c, the same method used for formulation10a was repeated except that a hydrophobic ethylene/vinyl acetatecopolymer emulsion (50% solids) (product of Harlow Chemicals marketedunder the trade name DM137) was used instead of Flexbond 150, 0.05 partsof sodium nitrate were added with the potassium chloride and the partsby weight were changed to the figures given in Table 6.

[0100] To prepare formulations 10d and 10e, the same method used forformulation 10b was repeated except that solution A as described inExample 1 was used instead of solution G and the parts by weight werechanged to the figures given in Table 6.

[0101] To prepare formulations 10f and 10g, the same method used forformulation 10d was repeated except that potassium chloride was omittedand the parts by weight were changed to the figures given in Table 6.TABLE 6 COMPOSITION by WEIGHT Formulation 10a 10b 10c 10d 10e 10f 10g58% NaAMPS 44 44 65 35 35 35 37 KCl 4 5 5 5 5 SPA 20 20 10 25 25 15 18Glycerol 20 20 23 20 20 30 30 Gum Arabic DM 137 15 2 15 15 15 10Flexbond 150 15 PEG 600 3 10 10 5 5 Sodium Nitrate 0.05 PI/XL 0.13 0.130.15 0.12 0.13 0.15 0.15 (Solution) (G) (G) (G) (A) (A) (A) (A) G′(@ 1rad/s) 6156 4756 G′(@ 100 rad/s) 15219 15412 G″(@ 1 rad/s) 1775 1840G″(@ 100 5748 7743 rad/s)

[0102] To prepare formulations 10h, 10i and 10j, the same method usedfor formulation 10g was repeated except that the parts by weight werechanged to the figures given in Table 7.

[0103] To prepare formulations 10k, 10l and 10m, the same method usedfor formulation 10j was repeated except that a propylene oxide/ethyleneoxide block copolymer surfactant (designated PE/F127 and manufactured byBASF) was added with the glycerol and the parts by weight were changedto the figures given in Table 7. TABLE 7 COMPOSITION by WEIGHTFormulation 10h 10i 10j 10k 10l 10m 58% 37 35 35 35 35 35 NaAMPS SPA 1815 25 25 25 25 Glycerol 30 33 20 20 20 20 DM 137 10 10 15 15 15 15 PEG600 10 5 10 10 10 10 PE/F127 1 5 9 PI/XL 0.15 (A) 0.15 (A) 0.14 (A) 0.14(A) 0.14 (A) 0.14 (A) (Solution)

[0104] As will be seen, the invention presents a number of differentaspects and it should be understood that it embraces within its scopeall novel and inventive features and aspects herein disclosed, eitherexplicitly or implicitly and either singly or in combination with oneanother. Also, many detail modifications are possible and, inparticular, the scope of the invention is not to be construed as beinglimited by the illustrative example(s) or by the terms and expressionsused herein merely in a descriptive or explanatory sense.

1. A bioadhesive composition comprising: (i) a water activity in therange of 0.4 to 0.9; (ii) an elastic modulus at 1 rad/s in the range of700 to 15,000 Pa; (iii) an elastic modulus at 100 rad/s in the range of2000 to 40,000 Pa; (iv) a viscous modulus at 1 rad/s in the range of 400to 14,000 Pa; and (v) a viscous modulus at 100 rad/s in the range of1000 to 35,000 Pa; wherein the viscous modulus is less than the elasticmodulus in a frequency range of 1 to 100 rad/s.
 2. A bioadhesivecomposition according to claim 1 wherein the impedance of saidcomposition at 500 MHz is less than 10 ohms.
 3. A bioadhesivecomposition according to claim 1 which comprises an aqueous plasticiser,a copolymer of a hydrophilic unsaturated water-soluble first monomer, ahydrophilic unsaturated water-soluble second monomer, and across-linking agent, wherein said first monomer enhances the bioadhesiveproperties of said composition.
 4. A bioadhesive composition accordingto claim 1 which is obtained by polymerising an aqueous reaction mixturecomprising a hydrophilic unsaturated water-soluable first monomer, ahydrophilic unsaturated water-soluble second monomer, and across-linking agent, wherein said first monomer enhances the bioadhesiveproperties of the composition.
 5. A bioadhesive composition according toclaim 3 wherein said first monomer enhances the mechanical strength ofsaid composition and/or said second monomer increases the water activityof said composition.
 6. A bioadhesive composition according to claim 5wherein said second monomer lowers the electrical impedance of saidcomposition and enhances the electrical conductivity of saidcomposition.
 7. A bioadhesive composition according to claim 3 whereinsaid first monomer is a compound of formula

wherein R¹ is an optionally substituted hydrocarbon moiety, R² ishydrogen or an optionally substituted methyl or ethyl group, and M ishydrogen or a cation.
 8. A bioadhesive composition according to claim 7wherein R¹ is an optionally substituted alkyl, cycloalkyl or aromaticmoiety containing from 3 to 12 carbon atoms.
 9. A bioadhesivecomposition according to claim 7 wherein R¹ is

wherein R³ is hydrogen or an optionally substituted straight or branchedchain alkyl group possessing from 1 to 6 carbon atoms and R⁴ is anoptionally substituted straight or branched chain alkyl group possessingfrom 1 to 6 carbon atoms.
 10. A bioadhesive composition according toclaim 3 wherein said second monomer is a compound of formula

wherein R⁵ is hydrogen or optionally substituted methyl or ethyl, R⁶ ishydrogen or a cation and R⁷ is an optionally substituted alkylene moietyof 1 to 4 carbon atoms.
 11. A biomedical electrode which comprises abioadhesive composition according to claim 1 in association with anelectrically conductive interface.
 12. A biomedical electrode accordingto claim 11 which further comprises a support.
 13. A fixation productfor attaching a biomedical device to the human body, wherein saidproduct comprises a bioadhesive composition of claim 1.